This invention relates generally to fuel systems, and more specifically to aircraft fuel systems.
In aircraft fuel systems, fuel is commonly pumped from a fuel tank using a low pressure mechanical fuel pump receiving indirect mechanical power from a gas turbine engine drive train. The low pressure pump moves fuel through the fuel system and through various components of the system, such as a heat exchanger and a fuel filter. Downstream of the low pressure pump, it is common for a high pressure mechanical pump, also receiving indirect mechanical power from a gas turbine engine's drive train, to receive the fuel and discharge it at a pressure higher than the pressure created by the low pressure pump. Often downstream of the high pressure pump is a bypass line containing a bypass valve for bypassing excess fuel and routing the excess fuel back to the fuel tanks.
Also downstream of the high pressure pump is a branch where fuel can be sent down one of two paths. At the end of the first path are engine fuel nozzles, which spray fuel into combustors of a gas turbine engine. In the second path are servos which use the fuel as a hydraulic pressure source. Servos control many fluids within a turbine engine system such as compressor bypass air.
The mechanically controlled pumps operate at speeds related to engine speeds, because they are indirectly mechanically powered by the turbine engine. Therefore, as the engine speed increases, so does the speed of the low pressure and high pressure pump. Because of this, a bypass valve is required to prevent pressure and flow rates from exceeding operational maximums of the components served by fuel. An effect of this is that large pumps are used to ensure minimum flow rates are provided to fuel nozzles at minimum engine speeds and minimum operating pressure of the servo systems. While this system is operable, significant improvements may be made to reduce operating cost and increase efficiency.